Method of making dark heater coatings including tungsten

ABSTRACT

INSULATING PARTICLES OF ALUMINUM OXIDE, ZIRCONIUM OXIDE OR BERYLLIUM OXIDE ARE COATED WITH TUNGSTEN WHICH IS TURN IS OVERCOATED WITH PLATINUM TO SUBSTANTIALLY PREVENT THE TUNGSTEN FROM OXIDIZING AND CHANGING COLOR. IN ADDITION TO PLATINUM, IRIDIUM, ALUMINUM OXIDE OR ZIRCONIUM OXIDE CAN BE USED AS THE OXIDATION PREVENTING COATING. THE TUNGSTEN, PLATINUM, OR, IRIDIUM, ALUMINUM OXIDE OR ZIRCONIUM OXIDE ARE DEPOSITED FROM METALLIC SOAPS OF THE SAME. A PARTICULAR EXAMPLE OF SUCH A SOAP IS A METAL RESINATE.

United States Patent Office 3,737,342 Patented June 5, 1973 3,737,342 METHOD OF MAKING DARK HEATER COATINGS INCLUDING TUNGSTEN John J. Decker and Donald R. Kerstetter, Emporium, Pa., assignors to GTE Sylvania Incorporated, Seneca Falls, NY. No Drawing. Filed July 15, 1971, Ser. No. 163,050 Int. Cl. B44d [/14 US. Cl. 117-217 2 Claims ABSTRACT OF THE DISCLOSURE Insulating particles of aluminum oxide, zirconium oxide or beryllium oxide are coated with tungsten which in turn is overcoated with platinum to substantially prevent the tungsten from oxidizing and changing color. In addition to platinum, iridium, aluminum oxide or zirconium oxide can be used as the oxidation preventing coating. The tungsten, platinum, or, iridium, aluminum oxide or zirconium oxide are deposited from metallic soaps of the same. A particular example of such a soap is a metal resinate.

BACKGROUND OF THE INVENTION This invention relates to dark overcoats for improving the heat radiation of heaters for thermionic cathodes and more particularly to such coatings including tungsten wherein the tungsten is substantially prevented from oxidizing. Further, the dark overcoat employs far less tungsten than prior art devices which were subjected to oxidation. Heaters having a dark outer surface have been made which include metallic tungsten as an admixture with aluminum oxide in a second layer of material applied over a first layer of material such as aluminum oxide; however, these heaters can develop leakage paths to the oathode and, furthermore, can lose the dark color if the tungsten should oxidize, a frequent occurrence.

It would be advantageous and an advance in the art if a method of making a dark overcoated layer could be developed to provide a coating not having the disadvantages of the prior art.

OBJECTS AND SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide a method of making dark heater coatings that obviate the disadvantages of the prior art.

It is another object of the invention to enhance dark heater coatings.

Still another object of the invention is the provision of a method of making dark heater coatings Which include metallic tungsten which is protected from oxidation.

These objects are accomplished in one aspect of the invention by the provision of a method for making dark heater coatings which comprises the steps of applying to particles of electrically insulating material a layer of metallic tungsten. Over the tungsten is applied a layer of a different material which substantially protects the tungsten from oxidation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims.

Referring now to the invention with greater particularity, a dark heater overcoating material comprises particles of aluminum oxide, zirconium oxide or beryllium oxide having thereon a layer of tungsten amounting to about 2% to 2.5% by weight of the particles. Applied over the tungsten is a layer of a different material which substantially covers the tungsten, thereby protecting the same from oxidation, or alloys with the tungsten providing an oxidation resistance alloy. The different material is selected from the group consisting of platinum, iridium, aluminum oxide and zirconium oxide, with platinum being a preferred embodiment.

In the case of the platinum group metals, oxidation resistance is achieved by the platinum acting as a barrier coating to oxidation or by forming a platinum-tungsten alloy when the material is heated above the alloying temperature. The non-porous alloy thus formed is oxidation resistant.

The tungsten coated particles, which now weigh about tungsten resinate, as is the platinum or iridium. When one of the oxides of aluminum or zirconium is used they also can be deposited from aluminum or zirconium soaps because the aluminum or zirconium is oxidized in the air firing and will not reduce to the metal in the subsequent reduced atmosphere firing.

In the above-mentioned preferred embodiment, a particle having a 2% tungsten coating with a 1% platinum coating thereover is made as follows:

To 36.8 gms. of a tungsten soap, e.g. (tungsten resinate), containing 22.2% tungsten by weight, add ml. of toluene as a diluent. Then add 400 gms. of aluminum oxide (or zirconium oxide or beryllium oxide) very slowly and mix until the particles are uniformly coated with soap. Dry the coated particles at C. for 4 hours to evaporate the solvent (stirring occasionally with a spatula to prevent crust formation). After drying, the soap coated mixture is sieved through a 100 mesh sieve and fired in air at 500 to 550 C. for 1 hour to remove the organic portion of the soap. This firing leaves a layer of tungsten oxide on the particles. After the air firing the particles are fired in dry dissociated ammonia for 15 minutes at 875 C. to reduce the tungsten oxide to metallic tungsten.

The tungsten coated particles, which now weight about 408 gms. are now added to 17 gms. of a platinum soap containing 26% platinum by weight (e.g., platinum resinate) which has been diluted with 100 ml. of toluene. Again, the particles are added slowly and mixed until they are uniformly coated with the soap. Following, the abovedescribed steps of dry-sieving, firing in air and firing in dissociated ammonia are performed. While it is probable that the platinum coats out as the metal rather than the oxide, it is possible that the air firing could oxidize some of the tungsten, therefor the final firing will reduce any oxides so formed to the metal.

The particles can have a 2 /2% tungsten layer and a 1 /2% platinum layer applied by the same technique by using 45 gms. of tungsten soap and 24.5 gms. of platinum soap. Suitable soaps for either embodiment are Englehard #8629 Tungsten Resinate and Englehard #9450 Platinum Resinate.

The tungsten-platinum coated particles can be applied over a previously aluminum oxide layer by dip coating. A suitable dip coating suspension can be made as follows:

In a pebble mill mix:

300 gms. W-Pt. coated A1 0 200 ml. butyl acetate 200 ml. 3.5% solution of 60 seconds nitrocellulose lacquer in amyl acetate.

Mill for about 15 hours at 60 r.p.m. It will be seen from the above that there is herein provided a new and novel method of manufacturing dark heater coatings. The 2%-2 /z% by weight of tungsten is much less than that used in the prior art admixtures (as much as 50% by weight tungsten) and deposits much more uniformly than the aforementioned admixtures. The tungsten is substantially protected from oxidation by the layer of material applied thereover and the method of applying this layer is simple to use. It is preferred to apply the different material layer from a metallic soap.

While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

We claim:

1. In a method of forming particles for use as a dark overcoat on electron discharge device heaters, the steps comprising: applying to particles of insulating material selected from the group consisting of aluminum oxide, zirconium oxide and beryllium oxide a layer of metallic tungsten amounting to about 2% to 2.5% by weight of said particles, said particles being coated by adding about 400 gmS. of said particles to a tungsten resinate solution diluted with about 100 ml. of toluene; mixing the same to uniformly coat said particles; drying said coated particles at about 110 C. for about 4 hours to evaporate said diluent; sieving said coated particles through a 100 mesh sieve; firing said coated particles in air at about 500 C. v

to 550 C. for about 1 hour to remove the organic portion of said resinate and, in the case of tungsten, to oxidize the same; and then firing said coated particles in dissociated ammonia for about 15 minutes at about 875 C. to reduce said tungsten oxide to metallic tungsten; and applying a layer of a diiferent material over said tun'gsten whereby oxidation of said tungsten is substantially prevented, said difi'erent material being selected from the group consisting of platinum, iridium, aluminum oxide and zirconium oxide.

2. The method of claim 1 wherein said dilferent material is applied from a resinate.

References Cited UNITED STATES PATENTS 3,051,592 8/ 1962 Woerner 117-227 3,189,482 6/1965 Bajars et al 117227 3,195,004 7/1965 Hassett 1172l7 3,490,944 1/1970 Almer et al 117-221 3,500,454 3/1970 Emerick 313337 3,553,521 1/1971 Bakker 313337 2,853,398 9/1958 Mackiw et al 117-100 B 3,192,042 6/1965 Spacil 117100 B CAMERON K. WEIFFENBACH, Primary Examiner US. Cl. X.R. 

